The OFDM modulation method is a modulation method applies an inverse Fourier transform to 2n primary modulated (QPSK, 16ASAM, etc.) transmitted signal symbols to form 2n number of subcarriers orthogonal to each other on the frequency axis.
In a wireless communication system adopting such a OFDM modulation system, a transmitting side subjects transmission data to serial-parallel conversion and performs an inverse fast Fourier transform (IFFT) on the converted data to collectively modulate the plurality of subcarriers orthogonal to each other.
At the transmitting side, a burst signal serving as a synchronization training signal referred to as a preamble signal is added to the header of a modulated signal having a frame structure subjected to such IFFT process and transmitted.
Then, at the receiving side, processes such as automatic gain control (AGC), frequency offset correction, and FFT (Fast Fourier Transform) timing generation are performed using the preamble signal, whereby an FFT operation is performed on the basis of the generated FFT timing.
Note that in a receiver of a wireless communication system, since a reception signal level needs to be adjusted in the dynamic range of an A/D converter, the receiver is equipped with an AGC circuit as the circuit for adjusting the reception signal level to within the dynamic range of the A/D converter.
The AGC circuit synchronizes a timing to within a period of this burst signal, while controls the amplification gain based on the basis of the received level of the burst signal.
Further, in the receiver of the wireless communication system adopting the OFDM demodulation method, the timing for performing the FFT process to a reception symbol needs to be optimized.
This is because offset of the FFT timing leads to inter-symbol interference (ISI) or symbol rotation and therefore leads to the deterioration of the receiving performance.
The FFT timing is set by utilizing the above explained burst signal (training signal) referred to as a preamble added to the header of transmission data.
In the past, the FFT timing was set based on the point a correlation result exceeded a threshold value at the preamble portion using an auto-correlation or cross-correlation circuit.
Note that the auto-correlation is for finding the correlation between repetitive signals included in the preamble portion.
On the other hand, the cross-correlation is for obtaining the correlation between a data sequence known in advance and input data sequence.
Generally, the auto-correlation is not much affected by reflection and fading, but has a weak point in that it shows the correlation of even data and noise other than the preamble.
On the other hand, the cross-correlation does not detect the correlation of noise and unrelated data, however, when there is a large offset in a reception frequency and a reception wave is changed due to reflection and fading, a peak of the correlation tends to become small.
In this manner, since the auto-correlation and cross-correlation are affected by the reflection at a channel, S/N, etc., in the above explained method of using the threshold value for generating the FFT timing, there is encountered a disadvantage that a low threshold value that can be used commonly under various transmission conditions must be set and detecting an accurate timing is difficult.
Further, among the wireless LAN systems using the 5 GHz recently standardized, time division multiple access (TDMA) is adopted in the Wireless 1394 and HiperLAN/2.
In the TDMA wireless communication system, the frame synchronization is the most basic item, however, it has problems such as the following:
(1) In wireless communications, synchronization of every frame cannot always be detected due to the effects of channel conditions such as the occurrence of the above explained fading.
(2) In the above-mentioned system of 5 GHz, to make the system inexpensive, not a high precision crystal oscillator TCXO with temperature compensation, but crystal is used. On account of this, a reference frequency offset between a base station and a mobile station becomes a maximum 40 ppm. This means that the frequency shifts by four clocks in 100,000 clocks. Depending on the frame period, if this offset is not corrected well, the frame synchronization will easily be lost.
(3) If the frame synchronization is lost, more than the usual number of frames will be needed to obtain another synchronization. During that time, a large amount of data transfer will be interrupted.
In a best effort system, transferring the data again will do, however, this becomes a fatal problem if there is a desire to guarantee a certain degree of QoS (Quality of Service).
(4) In the Wireless 1394 system, due to connection with the Wireless 1394, synchronization with a system having a large offset (100 ppm) is necessary, and a frame synchronization system having good compliance is required.